High Order Filter Circuit

ABSTRACT

A high order filter circuit is integrated by a plurality of the low order filter circuits. Before correcting the high order filter circuit, switch units may restore the high order filter circuit to the low order filter circuits for correction, and then combine the corrected low order filter circuits to form the original high order filter circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/627,829 filed on Jun. 20, 2017, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to signal processing technologies, and moreparticularly, to a low order filter circuit having a frequencycorrection function, a frequency correction method for the low orderfilter circuit, and a high order filter circuit.

Descriptions of the Related Art

Telemedicine is getting more widely applied in non-clinical fields dueto rapid development of transmission technology, wireless communicationtechnology and other associated technologies. A telemonitoring systemfor patients in a telemedical care system primarily provides home careservices for patients with chronic diseases, making them more activelyinvolved in their self-health management. By these technologies,physiological conditions such as ECG signal, blood pressure, bodytemperature, blood sugar and other physiological messages of a homemedical care user are transmitted through networks to a central databasefor storage and for establishing a personal physiological database. Aslong as there is an abnormal change of the physiological messages, awarning signal is generated and a medical treatment proceeds.

The above telemdical care system when actually measuring signals isusually affected by noise interferece on human body or wire sensing,thereby causing signal distortion and deteriorating wire efficacy. Anotch filter is thus added in a sensing channel to filtering offinterfering noises, such as mains signals. Alternatively, a band-passfilter is used to retrieve required signal bands and output signals tobe retrieved, such as R wave signals in ECG signals or channel signalsin a communcation system. FIGS. 1 and 2 show a sencing channel filtercircuit of a conventional telemdical care system, that is, an analogfront end circuit in the example of a second order filter circuit. Abasic structure of the sencing channel filter circuit is shown in FIG.1, which includes: a preamplifier 10, a low-pass filter 11, a notchfilter 12, a post-amplifier 14, and an A/D converter 15. FIG. 2 differsfrom FIG. 1 in having a band-pass filter 13 in place of the notch filter12 in FIG. 1. Signals are firstly amplified by the preamplifier 10, andthen high frequency noises and interfering signals are filtered offtherefrom by the low-pass filter 11 and the notch filter 12 (orband-pass filter 13) respectively. The post-amplifier 14 amplifiesamplitudes of the retrieved signals that are the outputted to the A/Dconverter 15 where the retrieved signals are converted to digitalsignals, which then undergo DSP (digital signal processing) by a backend of the telemdical care system.

However, a center frequency of the notch filter 12 or band-pass filter13 shown in FIGS. 1 and 2 is affected to have deviation by process (P),voltage (V), temperature (T) and usage time. In this case, the filters12 and 13 cannot effectively filter off interfering noises or retrievedesired signals, and signals thus being sensed would encouter unexpectedattenuation.

Therefore, how to propose a new filtering technology using the aboveanalog front end circuit as a sensing channel to effectively correct thecenter frequency of the notch filter 12 or band-pass filter 13 is animportant topic in the art.

SUMMARY OF THE INVENTION

In view of the above drawbacks of the conventional technology, a primaryobject of the invention is to provide a low order filter circuit havinga frequency correction function, a frequency correction method for thelow order filter circuit, and a high order filter circuit, which mayperform frequency correction on a notch filter or a band-pass filter inthe circuit, so as to eliminate deviation of a center frequency of thenotch or band-pass filter caused by influence from process, voltage,temperature and usage time in the conventional technology.

Another object of the invention is to provide a new high order filtercircuit, which is made up of the low order filter circuits of theinvention in the form of a cascade or ladder-type structure, and isapplied in a telemdical care system to prevent unexpected attenuation ofsignals from being sensed by the system.

To achieve above object and other object, the invention is to provide alow order filter circuit having a frequency correction function,including: a second order filter unit for receiving an inputted signaland filtering the inputted signal to retrieve a predetermined bandsignal and output the predetermined band signal; an analog-to-digitalconverter (ADC) having a first working status and a second workingstatus, for detecting a peak of the predetermined band signal from thesecond order filter unit and digitalizing the peak when the ADC is inthe first working status, and for detecting and converting thepredetermined band signal from the second order filter unit to a digitalsignal and outputting the digital signal when the ADC is in the secondworking status; and a digital correction unit for comparing thedigitalized peak with a default value and generating a comparisonresult, and according to the comparison result, the digital correctionunit generating a frequency control signal and a working status controlsignal and sending them as feedbacks respectively to the second orderfilter unit for adjusting its working frequency and to the ADC forswitching its working status.

Preferably, the second order filter unit of the invention is a secondorder notch filter unit or a second order band-pass filter unit, and theworking frequency is a notch center frequency of the notch filter unitor a band-pass center frequency of the band-pass filter unit.

Preferably, the second order notch filter unit of the invention includesa preamplifier, a low-pass filter, a notch filter and a post-amplifier,which are connected in a cascade.

Preferably, the second order band-pass filter unit of the inventionincludes a preamplifier, a low-pass filter, a band-pass filter and apost-amplifier, which are connected in a cascade.

Preferably, the second order filter unit of the invention is made up ofan operational transconductance amplifier (OTA). The working frequencyof the second order filter unit is adjusted by changing transconductanceof the OTA.

Preferably, the digital correction unit of the invention includes: aregister unit for storing the digitalized peak; a comparison unit forcomparing the digitalized peak with the default value and generating acomparison result; a control unit for generating the working statuscontrol signal and a counting mode control signal according to thecomparison result from the comparison unit; and a counting unit forperforming forward or backward counting according to the counting modecontrol signal from the control unit to generate the frequency controlsignal.

The invention further provides a frequency correction method for the loworder filter circuit, including the steps of: in step S1, setting aninitial frequency control signal via the counting unit, and with theinitial frequency control signal, allowing the ADC to detect a peak of apredetermined band signal outputted from the second order filter unitand digitalize the peak to form a first digitalized peak that is storedin the register unit and serves as the default value; in step S2, usingthe counting unit to continuously form a next frequency control signal,and with this frequency control signal, allowing the ADC to detect apeak of a predetermined band signal outputted from the second orderfilter unit and digitalize the peak to form a second digitalized peakthat is outputted to and stored in the register unit; in step S3, havingthe comparison unit compare the second digitalized peak formed in stepS2 with the first digitalized peak formed in step S1 and generate acomparison result, wherein if the comparison result matches apredetermined setting then step S4 is executed, the control unitoperates to have the counting unit stay in a current counting modewithout changing its counting direction and then to generate and send aworking status control signal to the second order filter unit foradjusting its working frequency, and the comparison unit keepsperforming comparison; and if the comparison result does not match thepredetermined setting then step S5 is executed, the control unitoperates to change the counting mode of the counting unit in a way ofchanging its counting direction to count reversely and then to generateand send a working status control signal to the second order filter unitfor adjusting its working frequency then step S6 is executed; in stepS6, determining if a number of times for the counting unit to change itscounting mode has reached a predetermined number, if yes, ending themethod; if no, returns step S3.

Preferably, in step S2 of the frequency correction method for the loworder filter circuit, the second order filter unit is a second ordernotch filter unit, and in the step of having the comparison unit comparethe second digitalized peak formed in step S2 with the first digitalizedpeak formed in step S1, if the second digitalized peak is smaller thanthe first digitalized peak, it means the comparison result matches thepredetermined setting; or the second order filter unit is a second orderband-pass filter unit, and in the step of having the comparison unitcompare the second digitalized peak formed in step S2 with the firstdigitalized peak formed in step S1, if the second digitalized peak islarger than the first digitalized peak, it means the comparison resultmatches the predetermined setting.

The invention further provides a high order filter circuit. The highorder filter circuit proposed in the invention is made up of the loworder filter circuits, including: a plurality of second order filterunits for filtering inputted signals; a plurality of switch units forconnecting the plurality of second order filter units in a cascade toform a high order filter unit when the switch units are closed, and forrestoring the high order filter unit to the plurality of second orderfilter units when the switch units are opened; an analog-to-digitalconverter (ADC) having a first working status and a second workingstatus, for detecting peaks of predetermined band signals outputted fromthe second order filter units and digitalizing the peaks when the ADC isin the first working status, and for detecting and converting thepredetermined band signals from the second order filter units to digitalsignals and outputting the digital signals when the ADC is in the secondworking status; and a digital correction unit for comparing thedigitalized peaks with a default value and generating comparisonresults, and according to the comparison results, the digital correctionunit generating frequency control signals and working status controlsignals and sending them as feedbacks respectively to the second orderfilter units for adjusting their working frequencies and to the ADC forswitching its working status.

In summary, the invention may correct a working frequency of a secondorder filter unit in the low order filter circuit. If the second orderfilter unit is a notch filter unit, the working frequency is its notchcenter frequency. If the second order filter unit is a band-pass filterunit, the working frequency is its band-pass center frequency. This mayeliminate the drawbacks caused by center frequency deviation of a notchor band-pass filter in the conventional notch or band-pass filtercircuit.

Furthermore, the high order filter circuit proposed in the invention ismade up of the low order filter circuits having a frequency correctionfunction in the form of a cascade or ladder-type structure. By thisstructure, switch units in the high order filter circuit may divide itinto a plurality of the low order filter circuits for which frequencycorrection can be performed through the frequency correction method ofthe invention. After the frequency correction is done, the switch unitsmay connect all the second order filter circuits to form the originalhigh order filter circuit for signal detection. The inventionsignificantly simplifies frequency correction operations for filterunits (notch or band-pass filter units) in the high order filtercircuit, and greatly improves integrity of the high order filtercircuit, thereby achieving desirable effects such as delicate designingand power saving. Moreover, the high order filter circuit of theinvention can be widely applied in any product or system requiring highsignal quality, for example, telemedical care system, so as to avoidunexpected attenuation of signals sensed by the system.

The objects, technical disclosures and features of the invention andeffects achieved thereby would be more clearly understood from thefollowing detailed description of the preferred embodiments taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1 and 2 are schematic diagrams showing basic structures of aconventional second order filter circuit.

FIG. 3 is a structural schematic diagram showing a low order filtercircuit having a frequency correction function according to theinvention.

FIGS. 4 and 5 are schematic diagrams showing preferred embodiments ofthe low order filter circuit of FIG. 3.

FIGS. 6 and 8 are flow charts of a frequency correction method for a loworder notch filter circuit and for a low order band-pass filter circuitrespectively according to the invention.

FIGS. 7 and 9 are schematic diagrams respectively showing deviation of acenter frequency of a second order notch filter circuit and of a secondorder band-pass filter circuit according to the invention.

FIGS. 10 and 11 are respectively an RLC structure circuit diagram of thelow order filter circuit and a circuit diagram of a second order OTAfilter circuit equivalent to the RLC circuit according to the invention.

FIG. 12 is a circuit diagram of a sixth order ladder-type notch filteraccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. The invention may, however,be embodied in many different forms and should not be construed as beinglimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the invention to those skilled in theart. In the drawings, the shapes and dimensions of elements may beexaggerated for clarity, and the same reference numerals will be usedthroughout to designate the same or like components.

The invention provides a low order filter circuit having a frequencycorrection function, a frequency correction method for the low orderfilter circuit, and a high order filter circuit formed by the low orderfilter circuits. In the invention, the low order filter circuit and highorder filter circuit include at least a notch or band-pass filter unit.In order to prevent a working frequency of the notch or band-pass filterunit (that is, a notch center frequency of the notch filter unit or aband-pass center frequency of the band-pass filter unit) from deviatingfrom a reference frequency as being affected by process, temperature,voltage and usage time, the following embodiments are depicted by addingdigital correction technology to a conventional filter circuit, so as toadjust the working frequency of the notch or band-pass filter unit, andthereby filter off interfering noises from inputted signals or desirablyretrieve required inputted signals.

FIG. 3 shows a basic structural schematic diagram showing a low orderfilter circuit having a frequency correction function according to theinvention. Referring to FIG. 3, the low order filter circuit includes: asecond order filter unit 20, an ADC (Analog-to-Digital converter) 21,and a digital correction unit 22, which are to be described in detail asfollows.

The second order filter unit 20 is used to perform a filtering processon an inputted signal (analog inputted signal) to filter off interferingnoises from the inputted signal and retrieve a predetermined bandsignal.

The ADC 21 is connected to the second order filter unit 20 and has apeak detection function operating at first and second working statuses.In the first working status, the ADC 21 may detect a peak of thepredetermined band signal and digitalize the peak, and output thedigitalized peak. In the second working status, the ADC 21 may convertthe predetermined band signal to a digital signal and output the digitalsignal.

The digital correction unit 22 is used to compare the digitalized peakoutputted from the ADC 21 with a default value, and generate a frequencycontrol signal according to the comparison result and send it as afeedback to the second order filter unit 20 for adjusting on its workingfrequency. The digital correction unit 22 is further used to generate aworking status control signal and send it as a feedback to the ADC 21for switching its working status.

FIGS. 4 and 5 show circuit structures of preferred embodiments of thelow order filter circuit of FIG. 3 respectively, wherein FIG. 5 differsfrom FIG. 4 in having a band-pass filter 204 in place of a notch filter202 of FIG. 4. As shown in FIGS. 4 and 5, the second order filter unit20 of FIG. 3 includes: a preamplifier 200, a low-pass filter 201, thenotch filter 202 (or the band-pass filter 204 shown in FIG. 5), and apost-amplifier 203. The above inputted signal is firstly amplified bythe preamplifier 200, and then high frequency noises and interferingsignals are filtered off therefrom by the low-pass filter 201 and thenotch filter 202 or band-pass filter 204 to obtain the predeterminedband signal. Then, the post-amplifier 203 also amplifies thepredetermined band signal to be outputted to the ADC 21. The ADC 21 mayperform the following processes on the predetermined band signal:detecting the peak of the predetermined band signal and digitalizing thepeak to be outputted to the digital correction unit 22, or convertingthe predetermined band signal to the digital signal to be outputted to asubsequent circuit.

Further referring to FIGS. 4 and 5, the digital correction unit 22 ofthe invention is used to perform comparison and determination on thedigitalized peak outputted from the ADC 21 so as to generate thefrequency control signal for controlling adjustment on the workingfrequency of the notch or band-pass filter and to generate the workingstatus control signal for controlling the ADC to switch its workingstatus. As shown in the figures, the digital correction unit 22includes: a register unit 220, a comparison unit 221, a control unit 222and a counting unit 223. The register unit 220 is used to store thedigitalized peak outputted from the ADC 21. The comparison unit 221 isused to compare the digitalized peak with the default value. The controlunit 222 is used to generate the working status control signal and acounting mode control signal according to the comparison result from thecomparison unit 221. The counting unit 223 is used to perform forward orbackward counting according to the counting mode control signal from thecontrol unit 222 so as to generate the frequency control signal and sendit as a feedback to the notch filter 202 in FIG. 4 or the band-passfilter 204 in FIG. 5, for allowing the notch filter 202 or band-passfilter 204 to adjust its notch center frequency or band-pass centerfrequency.

The invention also discloses a frequency correction method for the loworder filter circuit, which includes two correction methods respectivefor the second order notch filter unit and the second order band-passfilter unit. The correction method for the second order notch filterunit is to input a signal of reference frequency (that is, the aboveinputted signal) to the second order notch filter unit that is to becorrected. The second order notch filter unit retrieves a predeterminedband signal from the inputted signal and outputs it to the ADC 21 havinga peak detecting function where a peak of the predetermined band signaloutputted from the second order notch filter unit is detected anddigitalized. Then the digital correction unit 22 compares thedigitalized peak of the second order notch filter unit with apre-adjusted digital peak (considered as the default value), and makesdetermination to output a digital code (Xn) as a frequency controlsignal and send it as a feedback to the second order notch filter unitwhere adjustment is performed.

Moreover, the correction method for the second order band-pass filterunit is to input a signal of reference frequency (that is, the aboveinputted signal) to the second order band-pass filter unit that is to becorrected. The second order band-pass filter unit retrieves apredetermined band signals from the inputted signal and outputs it tothe ADC 21 having a peak detecting function where a peak of thepredetermined band signal outputted from the second order band-passfilter unit is detected and digitalized. Then the digital correctionunit 22 compares the digitalized peak of the second order band-passfilter unit with a pre-adjusted digital peak (considered as the defaultvalue), and makes determination to output a digital code (Xn) as afrequency control signal and sends it as a feedback to the second orderband-pass filter unit where adjustment is performed. Further, thedigital correction unit 22 generates a working status control signal andsends it as a feedback to the ADC 21 where its working status may beswitched. The working status switching process of the ADC 21 isperformed as follows: if the digital correction unit determinesaccording to the comparison result that the second order notch orband-pass filter unit has completed its center frequency correction, theworking status control signal from the digital correction unit allowsthe ADC 21 to switch to its second working status; otherwise, theworking status control signal from the digital correction unit allowsthe ADC 21 to stay in its first working status and carry out the signalpeak detecting function.

A correction method for a notch center frequency of the notch filter 202for the second order filter circuit in FIG. 4 is described in detailbelow. As shown in FIG. 6, the correction method includes the followingsteps.

First in step S1, the counting unit 223 sets a digital code (Xn) (forexample, a four-digits code) as 1000, which serves as an initialfrequency control signal. With the initial frequency control signal, theADC detects a peak of a predetermined band signal outputted from thesecond order notch filter unit, digitalizes the peak, and stores thedigitalized peak as a default value An−1 in the register unit 220 (thatis a peak digitalized value An−1 of the second order notch filter unitbefore any adjustment is performed on its notch center frequency). Thecontrol unit 222 generates a counting mode control signal (C) such as 0to allow the counting unit 223 to count up (that is, forward counting),so the next digital code Xn is 1001. Then, step S2 is executed.

In step S2, the counting unit 223 counts up, and the next digital codeXn is 1001. With this digital code, the second order notch filter unitperforms adjustment on its notch center frequency. The ADC 21 detects apeak of a predetermined band signal outputted from the second ordernotch filter unit after the frequency adjustment is done, anddigitalizes the peak and stores the digitalized peak An in the registerunit 220. Then step S3 is executed.

In step S3, the comparison unit 221 compares the digitalized peak Anwith the default value An−1. If An is smaller than An−1, it means thatthe outputted peak from the second order notch filter unit afteradjusting its notch center frequency tends to become smaller, so step S4is executed to continue correction of the notch center frequency in asingle direction. The control unit 222 outputs a control code C=0 (Keepmode) to the counting unit 223 where the digital code Xn is counted inthe same direction and sent to the second order notch filter unit, andmeantime the counting unit 223 is counting up (forward counting). Returnto step S3 where the comparison process is continued. If An is largerthan An−1, it means that the outputted peak from the second order notchfilter unit after adjusting its notch center frequency is gettinglarger. The digital correction unit 22 determines that the counting unit223 is counting in a wrong direction, and step S5 is executed such thatthe control unit 222 outputs a control code C=1 (Change mode) to allowthe counting unit 223 to count reversely (backward counting) the digitalcode and send it to the second order notch filter unit where adjustmentof the notch center frequency is performed.

In step S6, it determines if the number of times for the counting unit223 to switch its counting mode has reached a predetermined number. Ifyes, the correction method is completed. If no, the correction method iscontinued and returns to step S3. According to the frequency correctionmechanism of the invention, eventually three records of data wouldappear for back and forth adjustment. This then requires a mechanism tofix the notch center frequency being adjusted at a median value. In suchcase, the predetermined number of times of counting mode switching isset 3, such that when the counting unit 223 has switched its countingmode three times, the second order notch filter unit would complete itscorrection, that is, the correction method shown in FIG. 6 ends.

FIG. 7 shows deviation of the notch center frequency of the second ordernotch filter circuit during the correction process. Case 1 showscorrection being performed on the filter unit's notch center frequencythat has deviated towards a high frequency direction. Case 2 showscorrection being performed on the filter unit's notch center frequencythat has deviated towards a low frequency direction. Case 3 shows theshortest process of correcting the filter unit. These three types ofcases all stop when the counting unit has switched its counting modethree times. (Please note: dotted line L1 means timing of the countingunit 223 switching its counting mode; solid line L2 is frequencyresponse of the notch filter unit; bold arrow is frequency of referencesignal)

A correction method for the second order band-pass filter circuit isdescribed below with reference to FIGS. 5 and 8. The correction methodincludes the following steps.

First in step S11, the counting unit 223 sets a digital code (Xn) (forexample, a four-digits code) as 1000, which serves as an initialfrequency control signal. With the initial frequency control signal, theADC detects a peak of a predetermined band signal outputted from thesecond order band-pass filter unit, digitalizes the peak, and stores thedigitalized peak as a default value An−1 in the register unit 220 (thatis a peak digitalized value An−1 of the second order band-pass filterunit before any adjustment is performed on its band-pass centerfrequency). The control unit 222 generates a counting mode controlsignal (C) such as 0 to allow the counting unit 223 to count up (thatis, forward counting), so the next digital code Xn is 1001. Then, stepS12 is executed.

In step S12, the counting unit 223 counts up, and the next digital codeXn is 1001. With this digital code, the second order band-pass filterunit performs adjustment on its center frequency. The ADC 21 detects apeak of a predetermined band signal outputted from the second orderband-pass filter unit after the frequency adjustment is done, anddigitalizes the peak and stores the digitalized peak An in the registerunit 220. Then step S13 is executed.

In step S13, the comparison unit 221 compares the digitalized peak Anwith the default value An−1. If An is larger than An−1, it means thatthe outputted peak from the second order band-pass filter unit afteradjusting its frequency tends to become larger, so step S14 is executedto continue correction of the center frequency of the second orderband-pass filter unit in a single direction. The control unit 222outputs a control code C=0 (Keep mode) to the counting unit 223 wherethe digital code Xn is counted in the same direction and sent to thesecond order band-pass filter unit, and meantime the counting unit 223is counting up (forward counting). Return to step S13 where thecomparison process is continued. If An is smaller than An−1, it meansthat the outputted peak from the second order band-pass filter unitafter adjusting its center frequency is getting smaller. The digitalcorrection unit 22 determines that the counting unit 223 is counting ina wrong direction, and step S15 is executed such that the control unit222 outputs a control code C=1 (Change mode) to allow the counting unit223 to count reversely (backward counting) the digital code and send itto the second order band-pass filter unit where frequency adjustment isperformed.

In step S16, it determines if the number of times for the counting unit223 to switch its counting mode has reached a predetermined number. Ifyes, the correction method is completed. If no, the correction method iscontinued and returns to step S13. According to the frequency correctionmechanism of the invention, eventually three records of data wouldappear for back and forth adjustment. This then requires a mechanism tofix the adjusted center frequency of the second order band-pass filterunit at a median value. In such case, the predetermined number of timesof counting mode switching is set 3, such that when the counting unithas switched its counting mode three times, the band-pass filter unitwould complete its correction, that is, the correction method shown inFIG. 8 ends.

FIG. 9 shows deviation of the center frequency of the second orderband-pass filter circuit during the correction process. Case 1 showscorrection being performed on the filter unit's center frequency thathas deviated towards a high frequency direction. Case 2 shows correctionbeing performed on the filter unit's center frequency that has deviatedtowards a low frequency direction. Case 3 shows the shortest process ofcorrecting the filter unit. These three types of cases all stop when thecounting unit 223 has switched its counting mode three times. (Pleasenote: dotted line L1 means timing of the counting unit switching itscounting mode; solid line L2 is frequency response of the band-passfilter unit; bold arrow is frequency of reference signal)

The invention also proposes a high order filter circuit. The high orderfilter circuit is subject to transfer function analysis, and it is foundthat a notch center frequency of a high order notch filter or a centerfrequency of a high order band-pass filer is determined by multiple setsof parameter combinations. The invention thus utilizes the above loworder filter circuit and switch units to design the high order filtercircuit. The high order filter circuit includes: a high order filterunit, a plurality of switch units, an ADC and a digital correction unit.The high order filter unit includes a plurality of second order filterunits for filtering inputted signals. The plurality of switch units areused to, when they are closed, connect the second order filter units ina cascade to form the high order filter unit, and when they are opened,divide the high order filter unit into the plurality of second orderfilter units. The ADC has a first working status and a second workingstatus. When it is in the first working status, the ADC detects a peakof a predetermined band signal outputted from the second order filterunit and digitalizes the peak. When it is in the second working status,the ADC converts the predetermined band signal to a digital signal forbeing outputted. The digital correction unit is used to compare thedigitalized peak with a default value, and generate a frequency controlsignal and a working status control signal according to the comparisonresult and send them respectively as feedbacks to the second orderfilter unit for adjusting its working frequency and to the ADC forswitching its working status. When the high order filter unit isstructurally formed, during frequency correction, the switch units maywork in a way to divide the original high order filter unit intomultiple sets of second order filters (such as the second order filtersshown in FIGS. 3, 4 and 5). Then the correction method shown in FIG. 6or 8 proceeds to sequentially correct each set of the second orderfilters. When all the second order filters are corrected, the switchunits connect the second order filters to form the original high orderfilter unit for signal detection. The filter may be constructed bytransconductance-C(OTA-C). Frequency correction or adjustment isperformed by changing transconductance (Gin) of an operationaltransconductance amplifier (OTA) to change a notch or band-pass centerfrequency of the filter.

FIG. 10 is an RLC circuit diagram of the second order notch filter unitin the low order notch filter circuit shown in FIG. 4. FIG. 11 is acircuit diagram of a second order OTA notch filter unit formed by usingan OTA and capacitance to equivalently replace a passive component shownin FIG. 10. The invention integrates the above digital correctiontechnology and digital switches (that is, switch units described below)on the basis of the technologies shown in FIGS. 10 and 11 to form thehigh order notch filter circuit. For example, a sixth order ladder-tyeOTA notch filter circuit is shown in FIG. 12. For the circuit, equations(1.1), (1.2), (1.3), (1.4), (1.5) are listed out through KCL and KVL,allowing relation equations (1.6), (1.7), (1.8) to be derivated and thenintroduced into equation (1.5) to obtain equation (1.9), which is thensubject to transfer function to get equation (1.10).

$\begin{matrix}\left\{ \begin{matrix}{I_{0} = {v_{i} \cdot G_{m\; 0}}} \\{I_{1} = {v_{x} \cdot G_{m\; 1}}} \\{I_{2} = {v_{o} \cdot G_{m\; 2}}} \\{I_{x} = {\left( {v_{x} - v_{o}} \right) \cdot {sC}_{4}}}\end{matrix} \right. & (1.1) \\\left\{ \begin{matrix}{I_{3} = {v_{x} \cdot G_{m\; 3}}} \\{I_{4} = {v_{1} \cdot G_{m\; 4}}} \\{I_{5} = {v_{2} \cdot G_{m\; 5}}} \\{I_{6} = {{v_{1} \cdot G_{m\; 6}} = {v_{2} \cdot {sC}_{2}}}}\end{matrix} \right. & (1.2) \\\left\{ \begin{matrix}{I_{7} = {v_{x} \cdot G_{m\; 7}}} \\{I_{8} = {v_{3} \cdot G_{m\; 8}}} \\{I_{9} = {v_{o} \cdot G_{m\; 9}}} \\{I_{10} = {v_{3} \cdot G_{m\; 10}}}\end{matrix} \right. & (1.3) \\\left\{ \begin{matrix}{I_{11} = {v_{o} \cdot G_{m\; 11}}} \\{I_{12} = {v_{5} \cdot G_{m\; 12}}} \\{I_{13} = {v_{6} \cdot G_{m\; 13}}} \\{I_{14} = {{v_{5} \cdot G_{m\; 14}} = {v_{6} \cdot {sC}_{6}}}}\end{matrix} \right. & (1.4) \\\left\{ \begin{matrix}{I_{0} = {I_{1} + I_{4} + I_{x} + I_{8}}} \\{{I_{10} + I_{x}} = {I_{2} + I_{12}}}\end{matrix} \right. & (1.5) \\\left\{ \begin{matrix}{I_{4} = {v_{1} \cdot G_{m\; 4}}} \\{I_{3} = {{{v_{1} \cdot s}\; C_{1}} + I_{5}}} \\{{v_{x} \cdot G_{m\; 3}} = {{v_{1}{sC}_{1}} + {\frac{v_{1}G_{m\; 6}}{{sC}_{2}} \cdot G_{m\; 5}}}} \\{v_{1} = {v_{x}{G_{m\; 3}\left( \frac{1}{{sC}_{1} + \frac{G_{m\; 5}G_{m\; 6}}{{sC}_{2}}} \right)}}}\end{matrix} \right. & (1.6) \\\left\{ \begin{matrix}{I_{12} = {v_{5} \cdot G_{m\; 12}}} \\{I_{11} = {{{v_{5} \cdot s}\; C_{5}} + I_{13}}} \\{{v_{o} \cdot G_{m\; 11}} = {{v_{5}{sC}_{5}} + {\frac{v_{5}G_{m\; 14}}{{sC}_{6}} \cdot G_{m\; 13}}}} \\{v_{5} = {v_{o}{G_{m\; 11}\left( \frac{1}{{sC}_{5} + \frac{G_{m\; 13}G_{m\; 14}}{{sC}_{6}}} \right)}}}\end{matrix} \right. & (1.7) \\\left\{ \begin{matrix}{I_{7} = {{v_{3}{sC}_{3}} + I_{9}}} \\{{v_{x} \cdot G_{m\; 7}} = {{v_{3}{sC}_{3}} + {v_{o} \cdot G_{m\; 9}}}} \\{v_{3} = \frac{{v_{x} \cdot G_{m\; 7}} - {v_{o} \cdot G_{m\; 9}}}{{sC}_{3}}}\end{matrix} \right. & (1.8) \\\left\{ \begin{matrix}{{v_{i} \cdot G_{m\; 0}} = {{v_{x} \cdot G_{m\; 1}} + {\left( \frac{v_{x}G_{m\; 3}}{{sC}_{1} + \frac{G_{m\; 5}G_{m\; 6}}{{sC}_{2}}} \right)G_{m\; 4}} +}} \\{{\left( {v_{x} - v_{o}} \right) \cdot {sC}_{4}} + {\left( \frac{{v_{x} \cdot G_{m\; 7}} - {v_{o} \cdot G_{m\; 9}}}{{sC}_{3}} \right)G_{m\; 8}}} \\{{\left( \frac{{v_{x} \cdot G_{m\; 7}} - {v_{o} \cdot G_{m\; 9}}}{{sC}_{3}} \right)G_{m\; 10}} +} \\{{\left( {v_{x} - v_{o}} \right) \cdot {sC}_{4}} = {{v_{o} \cdot G_{m\; 2}} + {\left( \frac{v_{o}G_{m\; 11}}{{sC}_{5} + \frac{G_{m\; 13}G_{m\; 14}}{{sC}_{6}}} \right) \cdot G_{m\; 12}}}}\end{matrix} \right. & (1.9) \\{{T(s)} = {\frac{v_{o}}{v_{i}} = {\frac{D(s)}{N(s)} = \frac{{a_{0}\left( {s^{2} + c_{1}} \right)}\left( {s^{2} + c_{2}} \right)\left( {s^{2} + c_{3}} \right)}{{b_{6}s^{6}} + {b_{5}s^{5}} + {b_{4}s^{4}} + {b_{3}s^{3}} + {b_{2}s^{2}} + {b_{1}s} + b_{0}}}}} & (1.10)\end{matrix}$

With the use of software (Mathematica.7.0.0), some coefficients of thetransfer function are obtained, among which a0, b6, b5, b4, b3, b2, b1,b0, c3, c2, c1 are shown below.

 a₀ = G_(m 0)C₁C₂C₃C₄C₅C₆b₆ = C₁C₂C₃C₄C₅C₆(G_(m 1) + G_(m 2))b₅ = C₂C₆{C₃C₄C₅C_(m 3)G_(m 4) + C₁[C₄C₅(G_(m 7) − G_(m 9))(G_(m 8) − G_(m 10)) + C₃(C₅G_(m 1)G_(m 2) + C₄G_(m 11)G_(m 12))]}b₄ = C₃C₄C₅C₆G_(m 5)G_(m 6)(G_(m 1) + G_(m 2)) + C₂{C₁C₅C₆(G_(m 2)G_(m 7)G_(m 8) + G_(m 1)G_(m 9)G_(m 10))C₃[C₅C₆G_(m 2)G_(m 3)G_(m 4) + C₁(C₆G_(m 1)G_(m 11)G_(m 12) + C₄G_(m 13)G_(m 14)(G_(m 1) + G_(m 2)))]}b₃ = C₂C₆(C₅G_(m 3)G_(m 4)G_(m 9)G_(m 10) + C₁G_(m 7)G_(m 8)G_(m 11)G_(m 12)) + C₄(G_(m 7) − G_(m 9))(G_(m 8) − G_(m 10))(C₅C₆G_(m 5)G_(m 6) + C₁C₂G_(m 13)G_(m 14)) + C₃{C₅C₆G_(m 1)G_(m 2)G_(m 5)G_(m 6) + C₄C₆G_(m 5)G_(m 6)G_(m 11)G_(m 12) + C₂[C₆G_(m 3)G_(m 4)G_(m 11)G_(m 12) + G_(m 13)G_(m 14)(C₁G_(m 1)G_(m 2) + C₄G_(m 3)G_(m 4))]}b₂ = C₅C₆G_(m 5)G_(m 6)(G_(m 2)G_(m 7)G_(m 8) + G_(m 1)G_(m 9)G_(m 10)) + C₁C₂G_(m 13)G_(m 14)(G_(m 2)G_(m 7)G_(m 8) + G_(m 1)G_(m 9)G_(m 10)) + C₃{C₆G_(m 1)G_(m 5)G_(m 6)G_(m 11)G_(m 12) + G_(m 13)G_(m 14)[C₂G_(m 2)G_(m 3)G_(m 4) + C₄G_(m 5)G_(m 6)(G_(m 1) + G_(m 2))]}b₁ = C₆G_(m 5)G_(m 6)G_(m 7)G_(m 8)G_(m 11)G_(m 12) + C₃G_(m 1)G_(m 2)G_(m 5)G_(m 6) + C₄G_(m 5)G_(m 6)(G_(m 7) − G_(m 9))(G_(m 8) − G_(m 10)) + C₂G_(m 3)G_(m 4)G_(m 9)G_(m 10)b₀ = G_(m 5)G_(m 6)G_(m 13)G_(m 14)(G_(m 2)G_(m 7)G_(m 8) + G_(m 1)G_(m 9)G_(m 10))$c_{3} = \frac{G_{m\; 13}G_{m\; 14}}{C_{5}C_{6}}$$c_{2} = \frac{G_{m\; 7}G_{m\; 10}}{C_{3}C_{4}}$$c_{1} = \frac{G_{m\; 5}G_{m\; 6}}{C_{1}C_{2}}$

According to the derivated equations, the notch center frequency isdetermined by coefficients c1, c2, c3, wherein capacitance is set fixed.The notch center frequency can be adjusted effectively by partlyadjusting transconductance (Gm5, Gm6, Gm7, Gm10, Gm13, Gm14) of the OTA.It is found that zero c1, c2, c3 of the sixth order notch filter is madeup of three sets of second order notch filters.

Therefore, for correcting the notch center frequency of the sixth ordernotch filter as exemplified in the invention, it is to input a referencefrequency to the system, and allow the digital correction mechanismdescribed in the invention to firstly correct the notch center frequencyof each second order filter unit, that is to adjust the OTA thatreplaces equivalent capacitance. When each second order notch filterunit has been corrected, the switch units of the invention are used toconstruct the sixth order ladder-tye OTA notch filter shown in FIG. 12from the second order notch filter units.

The examples above are only illustrative to explain principles andeffects of the invention, but not to limit the invention. It will beapparent to those skilled in the art that modifications and variationscan be made without departing from the scope of the invention.Therefore, the protection range of the rights of the invention should beas defined by the appended claims.

What is claimed is:
 1. A high order filter circuit including: aplurality of second order filter units for filtering inputted signals; aplurality of switch units for connecting the plurality of second orderfilter units in a cascade to form a high order filter unit when theswitch units are closed, and for restoring the high order filter unit tothe plurality of second order filter units when the switch units areopened; an analog-to-digital converter (ADC) having a first workingstatus and a second working status, for detecting peaks of predeterminedband signals outputted from the second order filter units anddigitalizing the peaks when the ADC is in the first working status, andfor detecting and converting the predetermined band signals from thesecond order filter units to digital signals and outputting the digitalsignals when the ADC is in the second working status; and a digitalcorrection unit for comparing the digitalized peaks with a default valueand generating comparison results, and according to the comparisonresults, the digital correction unit generating frequency controlsignals and working status control signals and sending them as feedbacksrespectively to the second order filter units for adjusting theirworking frequencies and to the ADC for switching its working status. 2.The high order filter circuit according to claim 1, wherein the secondorder filter unit is a second order notch filter unit or a second orderband-pass filter unit, and the working frequency is a notch centerfrequency of the notch filter unit or a band-pass center frequency ofthe band-pass filter unit.
 3. The high order filter circuit according toclaim 2, wherein the second order notch filter unit includes apreamplifier, a low-pass filter, a notch filter and a post-amplifier,which are connected in a cascade.
 4. The high order filter circuitaccording to claim 2, wherein the second order band-pass filter unitincludes a preamplifier, a low-pass filter, a band-pass filter and apost-amplifier, which are connected in a cascade.
 5. The high orderfilter circuit according to claim 1, wherein the second order filterunit is made up of an operational transconductance amplifier (OTA). 6.The high order filter circuit according to claim 5, wherein the workingfrequency of the second order filter unit is adjusted by changingtransconductance of the OTA.
 7. The high order filter circuit accordingto claim 1, wherein the digital correction unit includes: a registerunit for storing the digitalized peaks; a comparison unit for comparingthe digitalized peaks with the default value and generating comparisonresults; a control unit for generating the working status controlsignals and counting mode control signals according to the comparisonresults from the comparison unit; and a counting unit for performingforward or backward counting according to the counting mode controlsignals from the control unit to generate the frequency control signals.